A variety of substituted acetylenic monomers that undergo a solid-state polymerization reaction which gives rise to color development, or other visually apparent change, in a predictable and irreversible manner, have long been used as active agents in time-temperature and other ambient condition indicators. Such indicators can provide a simple visual indication of the cumulative exposure of a host product to an environmental condition. They may be used to monitor the useful shelf life of perishable host products such as a foodstuff, vaccine, medicament or the like, which can be adversely affected by inappropriate ambient temperatures of their surroundings or storage environment. The indicator system may comprise a label affixed to the product or the product packaging or otherwise associated with the host product, or can be embodied in some other convenient form.
The color-forming, or other visual signaling reaction can be thermally or radiation induced, or may be induced by pressure, humidity, ambient gases or other factors. Many substituted diacetylenic monomers, for example diacetylenic urea compounds, show some sensitivity to both thermal and radiation stimuli. Some can be highly radiation sensitive, but are relatively insensitive to ambient thermal conditions, for example, 5,7-dodecadiyn-1,12 diol bis(n-octadecyl urethane), also referenced “4DOD” herein and 5,7-dodecadiyn-1,12 diol bis (n-butoxycarbonyl urethane).
Useful diacetylenic compounds, being insoluble in water and many organic solvents, are formulated as ink dispersions. Also, the ink dispersions typically are applied to suitable substrates, by printing or the like.
Desirably, the visual change exhibited by the indicator system, such as a change of color or color density, can be readily detected or determined by human or machine inspection, upon the elapse of a given time-temperature integral representing the product's shelf life. Some embodiments of the invention employ indicator agents that can exhibit a visual change in an indicator which is clearly discernible by the naked eye, for example changes in hue, value, intensity or opacity. Other embodiments can employ other indicator agents that require a further agency to reveal the change. For example, such other indicator agents may respond to heat or another monitored ambient condition with a change in fluorescence, magnetic resonance, or other non-visual property.
To the end of providing a clearly discernible visual change, known indicator systems can comprise a chemical composition appropriately formulated to provide a color change response which develops substantially concurrently with elapse of the given time-temperature integral so as to be useful for indicating expiration of a predesignated shelf life. The chemical composition may include a suitable active indicator agent, for example a polymerizable diacetylenic monomer, as is known in the art.
Useful examples of such time-temperature indicators include, for example, indicators available from assignee hereof, TEMPTIME Corporation, Morris Plains N.J., under the trademark FRESH-CHECK®. The FRESH-CHECK® indicators can be employed to indicate the freshness and safety of foods in supermarkets and elsewhere and can conveniently be embodied in an adhesive label which may readily be applied to a host product or, more commonly, to its packaging. It is also known to use comparable time-temperature indicators (“TTIs”) with appropriate algorithms to indicate the heat damage status of vaccines. For example, TEMPTIME Corporation supplies vaccine condition indicators which are at the date of this application specified for use by UNICEF in their vaccination programs.
Patel U.S. Pat. Nos. 3,999,946; 4,189,399 and 4,384,980 and Preziosi et al. U.S. Pat. Nos. 4,789,637 and 4,788,151 disclose thermally responsive, color-changing diacetylenic compounds which can be employed as indicator agents in time-temperature indicators. In response to heat, polymerization of the diacetylenes, occurring in the solid state provides a change in color. These compounds include, for example, ethyl-, propyl-, and octyl-substituted 2,4-hexadiyn-1,6-bis(alkylurea) compounds.
It is usually desirable to match the thermal response characteristics of the acetylenic monomer to the changes that are expected to occur in a given host product. Thus, the acetylenic monomer exhibits a desired visual change as a result of a time-temperature exposure that relates predictably to the anticipated shelf life of a product. For example, to indicate the end of the useful life of a vaccine vial, it may be desirable for an initially light-colored time-temperature indicator to appear dark after a predetermined time temperature integral which is satisfied by an exposure of 10 days at 90° F. or an equivalent exposure of say 20 days at 75° F. or the like. Fresh fish may have a shelf life of days in a refrigerated supermarket display, or of only a few hours if left out at room temperature. Other host products may have other requirements.
For example, commonly owned U.S. patent application Ser. No. 11/119,650 filed May 2, 2005, the entire disclosure of which application is incorporated by reference herein for all purposes, discloses a maturity indicator which can utilize a thermally sensitive acetylenic monomer to provide a visual indication of the maturity of a wide range of different host products including fruits, cheeses, meats, wines and so on. These host products have a variety of desirable maturation periods ranging from a few days for some fruits to a number of years for some wines at temperatures that may range from near freezing to relatively warm to temperatures that may be elevated above room temperature.
To meet these diverse monitoring needs it would be desirable to have a family of indicator materials that have a diversified range of performance parameters. For example, it may be desirable for the worker in the field to have an extensive range of color-changing thermally sensitive monomer materials from which to select an appropriate material whose response characteristics would precisely fit a given need. While a wide range of such compounds is available to serve different purposes, desired temperature sensitivity characteristics, appropriate for a particular host product, may not be readily available or may only be achievable with difficulty. Furthermore, only limited means are available for adapting, or tailoring, the thermal response properties of these materials to specific needs. While believed satisfactory for their intended purposes, these means may in some cases be unduly cumbersome or may not be adequate to meet every market need.
It is known that the range of color change or the composition reactivity can be varied by co-deposition, or co-crystallization, of different acetylenic compounds (at least one of which contains at least two conjugated acetylene groups) or by the co-deposition of one or more acetylenic compounds which contain at least two conjugated acetylene groups with one or more compounds which have similar molecular shape and polarity as the acetylenic compound, but which do not contain reactive acetylenic functionalities. Such co-depositions can be made from the vapor, melt or solution phases, or from combinations thereof.
Some polymerizable diacetylenic monomers are substantially thermally insensitive or inactive at or near room and other common ambient temperatures and are therefore not per se useful as time-temperature indicator agents for indicator inks that are responsive to ambient temperatures.
For example, Yee et al U.S. Pat. No. 4,215,208 discloses a number of polyacetylenes that exhibit reversible color changes at transition temperatures in the range of 180 to 220° C. These thermochromic polyacetylenes are described as being useful in temperature-indicator and indicia-display device applications. One example of such compounds is the aforementioned 5,7-dodecadiyn-1,12 diol bis(n-octadecyl urethane). Yee et al. do not appear to suggest their compounds could be useful in cumulative time-temperature indicators and indeed the described reversibility of the color-changing phenomenon is generally not a desirable property of a substance to be employed as an indicator agent for a cumulative time-temperature indicator.
Also, Roth U.S. Pat. No. 6,524,000 discloses recording materials useful for forming time-temperature indicators in a direct thermal imaging or printing apparatus. The recording materials employ diacetylenic compounds that are thermally inactive under normal storage and shipping conditions and which are heat-activated by a thermal print head, described as having an operating temperature of 50° C. to 250° C. (column 2, lines 63-65) being a temperature above ambient (column 5, line 67 to column 6, line 3). Roth suggests that inactive compounds may be converted to active compounds through heat activation, i.e. at the operating temperature, of an initiator compound, for example, a peroxide which thermally decomposes into free radicals.
Roth suggests polymerization enhancers can be used to increase the reactivity of acetylenic compounds “of the invention” (i.e. of the Roth invention) and that other compounds can decrease the reactivity (column 4, lines 4-12). Absent further relevant disclosure, it may be understood that the polymerization enhancer is intended to be effective during the heat activation process. Whether the polymerization enhancers are effective in increasing diacetylenic reactivity after heat treatment is not disclosed by Roth. Heat activation using a print head or the like is a cumbersome process imposing its own limitations and is inappropriate for many applications.
Other fields of polymer chemistry employ catalysts, polymerization initiators, accelerators and the like to control, and increase the rate of, polymerization reactions, including various peroxides and other reactive materials. These compounds may help provide polymerizable compositions such as adhesives, caulks, sealing agents, fillers and the like that have a range of reactivities. Examples of these compositions include silicone caulks, epoxy adhesives and polyester resins. However, caulks, sealants, fillers and like are unrelated to indicator agents employed to indicate environmental condition exposure history.
There may be difficulties in chemically controlling the polymerization rate of diacetylenes. Patel and Miller in Polymer Journal, Vol 13, pages 1075-1083 (1980) teach, on page 1075, righthand column, that polymerization of diacetylenes is initiated by radicals. In addition, these authors teach that radical initiators such as dicumyl peroxide inhibit rather than initiate polymerization because they do not form a solid solution and the radicals do not become adjacent to the triple bonds.
It is known in the art, for example from Patel et al. J. Polymer Sci: Poly Letters Ed, Vol 19, 511-517(1981) that diacetylenes (R—C≡C—C≡C—R, where R is a substituent group) polymerize in the solid state. According to Patel et al., “no catalysts previously have been found suitable for this reaction,” However, in a few acetylenes, polymerization was found by Patel et al. to have been accelerated by co-crystallization with other acetylenes. Otherwise, there was “no report on catalytic polymerization of diacetylenes” according to Patel et al. Furthermore, the reference teaches that initiation of polymerization by radical initiators such as peroxides (e.g. dicumyl peroxide) or azo compounds (e.g. 2,2′-azobisisobutylnitrile), would be difficult because it is difficult to incorporate solid or liquid initiators in solid diacetylenes. Still further, the paper teaches that even if one were able to incorporate such initiators in solid diacetylenes, they would be unlikely to work because it is unlikely that such large molecular radicals would be positioned suitably, relative to the rodlike diacetylene molecules, to initiate polymerization. Patel et al. further assert that such initiators or sensitizers may act as impurities and block polymerization rather than initiate it. To overcome such problems the paper suggests using chlorine gas to initiate the polymerization reaction. Chlorine is hazardous to personnel, corrosiive and, being gaseous, is difficult to handle. Accordingly, chlorine gas is unsuitable for most commercial purposes. Nor do Patel et al. describe any commercial applications of their gas treatment method which is apparently merely a report on research work-in-progress.
It may be expected that were the initiators to be unable to react with the diacetylene, for steric or other reasons, they could undergo side reactions that would negatively impact the appearance or physical properties of an indicator in which they were incorporated. Also, for applications where the indicator is to be formed into a film, for example to be printed on labels, it may be expected that initiator radicals might react with themselves to generate cage product impurities that could have a detrimental impact on film appearance. Furthermore, where indicators incorporating the diacetylene employ a film former, there is a possibility that unreacted initiator radicals might react with the film-former, causing it to cross-link and possibly become brittle.
JP Laboratories' International Publication Number WO 2004/077097, inventor Patel, discloses a diacetylene-based personal dosimeter. The dosimeter employs a radiation-sensitive ink which provides a radiation-induced color reaction to indicate cumulative dosage. According to the reference, shelf life extenders can be added to reduce thermal reactivity purportedly without affecting the radiation sensitivity of the coating. This is described as an advantage for the dosimeter application, reducing thermally induced darkening and facilitating monitoring of radiation-induced color. Disclosed shelf-life extenders include compounds such as heat stabilizers, quenchers and inhibitors of reactive species, radical and oxygen scavengers, antioxidants and the like. The inhibitory function of such compounds is the antithesis of reactivity-enhancement of thermally sensitive diacetylenic materials.
The foregoing description of background art may include insights, discoveries, understandings or disclosures, or associations together of disclosures, that were not known to the relevant art prior to the present invention but which were provided by the invention. Some such contributions of the invention may have been specifically pointed out herein, whereas other such contributions of the invention will be apparent from their context. Merely because a document may have been cited here, no admission is made that the field of the document, which may be quite different from that of the invention, is analogous to the field or fields of the present invention.